1. Technological Field
The present invention relates to an electrical switch device and more specifically to sealing a fluid conductor switch device.
2. Background Art
A reed relay is a typical example of a conventional small, mechanical contact type of electrical switch device. A reed relay has two reeds made of a magnetic alloy sealed in an inert gas inside a glass vessel surrounded by an electromagnetic driver coil. When current is not flowing in the coil, the tips of the reeds are biased to break contact and the device is switched off. When current is flowing in the coil, the tips of the reeds attract each other to make contact and the device is switched on.
The reed relay has problems related to large size and relatively short service life. As to the first problem, the reeds not only require a relatively large volume, but also do not perform well during high frequency switching due to their size and electromagnetic response. As to the second problem, the flexing of the reeds due to biasing and attraction causes mechanical fatigue, which can lead to breakage of the reeds after extended use.
In the past, the reeds were tipped with contacts composed of rhodium, tungsten, or were plated with rhodium or gold for conductivity and electrical arcing resistance when making and breaking contact between the reeds. However, these contacts would fail over time. This problem with the contacts has been improved with one type of reed relay called a xe2x80x9cwetxe2x80x9d relay. In a wet relay, a fluid conductor, such as mercury, is used to make the contact. This solves the problem of contact failure, but the problem of mechanical fatigue of the reeds remained unsolved.
In an effort to solve these problems, electrical switch devices have been proposed that make use of the fluid conductor in a channel between two switch electrodes. In the fluid conductor devices, the fluid conductor acts as the contacts connecting the two switch electrodes when the device is switched ON. The fluid conductor is separated between the two switch electrodes by a fluid non-conductor when the device is switched OFF. The fluid non-conductor fluid is generally high purity nitrogen or another such inert gas.
With regard to the size problem, the fluid conductor devices afford a reduction in the size of an electrical switch device since reeds are not required. Also, the use of the fluid conductor affords longer service life and higher reliability. However, the use of a gas fluid non-conductor presents sealing problems.
The fluid conductor devices are generally manufactured by joining together two substrates. For example, with past mercury switch devices, glass, silicon, or ceramic were used as the substrates and an adhesive agent or other resin used to join the substrates.
The substrate and the seal must both have low gas permeability, that is, good sealing properties. The sealing performance required of the fluid conductor switch devices can be grouped into two types: (1) short term sealing performance for intermittent conditions for suppressing leakage of gas due to sudden temperature-induced high internal gas pressures, and (2) long term sealing performance to prevent the slow exchange of the gas with the external atmosphere over an extended period of time.
The poor short-term sealing is linked to pronounced defects such as diminished switching efficiency or switch failure. The gas either will only partially separate the conductive fluid or fail to separate the conductive fluid entirely.
Short-term sealing problems usually do not occur under normal usage conditions, and there will be no short-term leakage of gas from the substrate itself unless the substrate is extremely porous or has pinholes, for example, with a diameter of several microns or more. Nor will there be any gas leakage over the short term from the adhesive or resin except in cases such as when the joint is imperfect or a gap remains at the joint interface.
Poor long-term sealing reduces long-term reliability. If the gas leaks through a seal and is replaced with air, oxygen and water vapor in the air will cause oxidation of conductive fluids or the electrodes. The oxidation can interfere with the contact between separated portions of the conductive fluid or can coat the electrodes to prevent proper contact of the electrodes with the conductive fluid.
Special packages (called xe2x80x9chermetic packagesxe2x80x9d) are used in fields that demand particularly high reliability, such as the aerospace industry, military applications, and some industrial semiconductor devices. Hermetic packages are often specified for high reliability semiconductor devices where it is important to minimize the direct contact of the semiconductor devices with the external atmosphere and the resulting reactions linked to degradation such as oxidation. The military standard or MIL Spec is well known as a definition of a hermetic seal and is defined as leakage of less than 1xc3x9710xe2x88x928 atmxc2x7cc/sec of helium through the seal.
It is commonly known that a hermetic seal is difficult to achieve by joining substrates with an adhesive or a resin. As a result, maintaining reliability over the long term requires the mounting of the fluid conductor switch device itself inside a separate hermetic package, which often includes a separate substrate and a glass lid sintered into place on the separate substrate over the fluid conductor switch device.
The separate hermetic packaging itself is generally expensive, and special equipment is required for sintering the lid on to the separate substrate. This substantially increases the cost of the doubled hermetic packages. Also, a hermetic package must be of a certain minimum size in order to ensure proper sealing. This means the doubled hermetic packages have large overall sizes, which reduce the size advantage compared to reed switch devices.
One attempt at avoiding this size problem has been to install an extremely large number of fluid conductor switch devices in a single, large hermetic package. However, there is a limit to the number of fluid conductor switch devices that can be incorporated before the hermetic sealing is lost.
In addition, the short-term sealing becomes a problem because of the proximity of the fluid conductor switch devices, which result in gas leakage from one device to another.
Solutions to these problems have been long sought but have long eluded those skilled in the art.
The present invention provides a switch device and a method of manufacture that includes providing two substrates. The two substrates collectively include a fluid conductor switch device structure and a trench surrounding the fluid conductor switch device structure. An inner seal material is deposited on one of the substrates and an outer seal material is deposited in the trench. The substrates are joined to one another using the inner seal material and a peripheral hermetic seal is formed between the substrates using the outer seal material. This results in a simple and inexpensive packaging method for manufacturing a device having a compact and relatively simple structure, but also has high operating reliability and a long service life.
Certain embodiments of the invention have other advantages in addition to or in place of those mentioned above. The advantages will become apparent to those skilled in the art from a reading of the following detailed description when taken with reference to the accompanying drawings.